(a) Field of the Invention
The present invention relates to a display panel, a light emitting display device using the display panel, and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display panel, a light emitting display device using the display panel, and a driving method thereof.
(b) Description of the Related Art
In general, an organic EL display panel is a display device for electrically exciting fluorescent and organic compounds and emitting light. The organic EL display panel voltage- or current-programs (M×N) organic emission cells to represent images. An organic emission cell includes an anode (e.g., an ITO: indium tin oxide), an organic thin film, and a metallic cathode layer. The organic thin film includes an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for balancing electrons and holes to improve emission efficacy, and additionally includes an electron injection layer) (EIL) and a hole injection layer (HIL).
Methods for driving the organic emission cells are classified as a passive matrix method, and an active matrix method using thin film transistors (TFTs). The passive matrix method provides anodes and cathodes that cross (or cross over or are perpendicular to) each other, and selects a line to drive the organic emission cells, while the active matrix method provides TFTs that access respective ITO pixel electrodes and drive the line according to a voltage maintained by a capacitance of a capacitor accessed to a gate of a TFT. Further, depending on formats of signals applied to the capacitor for establishing the voltage, the active matrix method can be categorized as a voltage programming method and a current programming method.
FIG. 1 shows an equivalent circuit diagram for a pixel circuit of a conventional voltage programming method. As shown in the organic EL display of the conventional voltage programming method, a transistor M1 is coupled to an organic EL element (OLED) to supply the current for emission, and the current of the transistor M1 is controlled by a data voltage applied through a switching transistor M2. A capacitor C1 for maintaining the applied voltage for a predetermined time is coupled between a source and a gate of the transistor M1.
When the switching transistor M2 is turned on, the data voltage is applied to the gate of the transistor M1 to charge the capacitor C1 with the voltage VGS at the gate and the source of the transistor M1, a current IOLED flows though the transistor M1 in correspondence to the voltage VGS, and the organic EL element OLED emits light in correspondence to the current IOLED.
The current flowing through the OLED is given as Equation 1.
Equation 1
      I    OLED    =                    β        2            ⁢                        (                                    V              GS                        -                          V              TH                                )                2              =                  β        2            ⁢                        (                                    V              DD                        -                          V              DATA                        -                                                        V                TH                                                            )                2            where IOLED is a current flowing through the OLED, VGS is a voltage between the gate and the source of the transistor M1, VTH is a threshold voltage of the transistor M1, VDATA is a data voltage, and β is a constant.
As given in Equation 1, the current corresponding to the data voltage is supplied to the OLED, and the OLED emits light corresponding to the supplied current. The applied data voltage has multiple-stage values within a predetermined range so as to represent gray scales.
The pixel circuit of the conventional voltage programming method has difficulties in obtaining high gray scales because of deviations of the threshold voltage VTH and the carrier mobility, the deviations being caused by non-uniformity of a manufacturing process. For example, in order to represent 8-bit (i.e., 256) gray scales in a case of driving thin film transistors by a voltage of 3V (volts), it is required to apply the voltage to the gate of the thin film transistor with an interval less than the voltage of 12 mV(=3 V/256), and if the deviation of the threshold voltage of the thin film transistor caused by the non-uniformity of the manufacturing process is 100 mV, it is difficult to represent high gray scales. Also, it becomes more difficult to represent high gray scales since the value of β in Equation 1 is differentiated because of the deviation of electron mobility.
The pixel circuit of the current programming method achieves uniform display characteristics when the driving transistor in each pixel has non-uniform voltage-current characteristics, providing that a current source for supplying the current to the pixel circuit is uniform throughout the whole panel.
However, the current flowing through the OLED is a fine (or small) current, and it accordingly needs a lot of time to charge a data line with the fine current. For example, it may require several milliseconds to charge the load of the data line with a fine data current of about several tens to several hundreds of nA assuming that the capacitance of the data line is about 30 pF. Thus, on considering a line charging time of several tens of μs , the conventional current programming method is insufficient.
Also, in the conventional current programming method, when the current flowing though the OLED is increased so as to reduce the time used for charging the data line, the total brightness of pixels is increased and image characteristics are degraded.